Wood is not a naturally durable material and will deteriorate by insect and fungal attack if left exposed to the environment. No wood species is immune from such attack, although variations in durability may occur according to the chemical or physical nature of individual species.
Durability may be imparted by limiting access of food, moisture or air to the invading organisms and there are many examples of dry and coated wood that have remained intact over centuries.
Alternatively wood durability may be imparted by incorporation of chemicals into the wood structure, which may be toxic or indirectly active against the invaders, e.g. by affecting reproductive capacity. Such chemicals are termed wood preservatives.
In many cases, surface treatment with active chemicals is insufficient for wood preservation because subsequent processing or fabrication may expose fresh surfaces or promote new pathways for biological attack. Thus deep penetration of actives is preferred to simple envelope treatment. Robust wood preservative systems incorporate either aqueous or non-aqueous solvent carriers to transport active chemical ingredients to the interior of the wood.
Aqueous wood preservative systems containing copper, chromium and arsenic (CCA) have been used commercially for over 60 years and proved highly effective. However CCA and recent replacements, also based mainly on copper salts, require a costly redrying process after treatment. Also customer acceptance of preservatives containing toxic or heavy metals may be limited and copper and chromium salts are highly coloured rendering them unsuitable for use in appearance grade wood products.
Organic solvent systems for wood preservation include both non-volatile and volatile solvents. Non volatile solvents, such as creosote, have been in commercial use since the nineteenth century but issues related to cleanliness, odour and paintability of the treated wood have led to a decline in the use of such treatments.
In contrast, treatments based on volatile solvents, which evaporate from wood over periods measured in days, have increased considerably over the last 30 years in New Zealand, where there are currently about 20 plants using such treatments. The process known as LOSP (light organic solvent process) utilizes white spirit, a petroleum fraction, as carrier for the active ingredients. The composition of white spirit differs according to refinery source but typically, as supplied in New Zealand, comprises 50% paraffins, 25% naphthenes and 25% aromatic hydrocarbons, has a flashpoint of 38° C., and a distillation range from 150°-200° C.
The major LOSP formulations used commercially in NZ and in other countries may contain insecticides such as permethrin and/or fungicides such as iodo propyl butyl carbamate (IPBC), or triazole compounds, such as tebuconazole or propiconazole or tin compounds such as tri butyl tin naphthenate. Additionally LOSP formulations may incorporate water repellents, such as hydrocarbon resins and waxes and may require use of co-solvents to aid dissolution of the active ingredients or water repellents.
The advantages of LOSP treatments are the ability to impregnate wood with water insoluble active ingredients; there is little swelling or grain raise of the treated wood, and the process is cost competitive because there is no requirement for re-drying of the wood. These advantages allow the treatment process to be the final stage of timber production and timber mills therefore avoid creating wastes (sawdust, shavings, off-cuts) containing treatment chemicals.
The disadvantages of LOSP treatments relate to issues of health, safety and the environment. Release of LOSP solvent from the treated wood to atmosphere denotes that the process is a significant contributor of volatile organic compounds (VOC's). Some of the hundreds of individual compounds which constitute white spirit have suspected carcinogenic status and some have undefined toxicological properties. White spirit has a characteristic odour and workers may suffer headaches and other symptoms when handling freshly treated timber.
In addition the process step of waiting for most of the solvent to be evolved from the timber before painting may take several weeks or more in colder climates or where minimal airflow occurs across wood surfaces. Long holding periods cause manufacturing bottlenecks and impose increased working capital costs.
Adverse health, safety and environmental properties of LOSP treatments may be mitigated by incorporating solvent capture and recycle as part of the process. Heating of the timber is required to effect fast removal rates of this only moderately volatile white spirit solvent (boiling point 150°-200° C.) from deep inside the wood. However at elevated temperatures, particularly above 60° C., the resin and fatty acid extractives fraction of the wood becomes increasingly soluble in the solvent, leading to unsightly “resin bleed” at the surface of the wood.
NZ Patent 535897 relates to a LOSP solvent recovery process based on radiofrequency (RF) heating of the wood to temperatures between 40° and 65° C. The ability of such a process to deal with resin bleed issues outlined above is not disclosed.
Alternative recoverable non-aqueous solvents are outlined by Richardson p 83 (Wood Preservation by Barry A Richardson, pub, Taylor and Francis, 1993) include LPG or Butane, known as the Drilon or Cellon Process and methylene chloride, known as the Dow process. In the case of the Cellon or Drilon process, where vacuum recovery is practised, the potentially hazardous conditions require difficult and high energy adiabatic restrictions, which have deterred commercialization. Methylene chloride use has also a number of health and safety concerns and the use of steam distillation recovery in the Dow process has a significant energy cost.
These limitations have prevented any widespread adoption of commercial development of solvent recovery for any of the organic solvent carrier wood preservation treatments.